1. Field of Invention
The invention relates to an abnormality detection system of an oxygen sensor used as an air fuel ratio sensor for a vehicle, and an abnormality detection method of the oxygen sensor.
2. Description of Related Art
It has been well known to set an air fuel ratio of air fuel mixture within a combustion chamber of an internal combustion engine at a desired value based on a concentration of oxygen contained in exhaust gas discharged from the combustion chamber, which is detected by an oxygen sensor. The oxygen sensor of critical current type, for example, has been conventionally used as the oxygen sensor of the above-described type. The oxygen sensor of critical current type includes a detection element formed of a solid electrolyte having inner and outer surfaces provided with electrodes, respectively. The inner surface of the detection element admits air to be introduced, and the outer surface of the detection element admits exhaust gas discharged from a combustion chamber. When voltage is applied between the electrodes of the detection element, it outputs a current (critical current) value corresponding to the oxygen concentration of the exhaust gas.
When the detection element of the oxygen sensor is deteriorated, or cracked owing to a certain cause, air that exists on the inner surface of the detection element is mixed with the exhaust gas that exists on the outer surface of the detection element. This may prevent the oxygen sensor from accurately detecting the oxygen concentration of the exhaust gas or the like. The process for applying negative voltage to the oxygen sensor has been generally used so as to detect the aforementioned abnormality in the oxygen sensor. In response to the application of the negative voltage, the oxygen sensor outputs a current value, based on which it is determined whether there is abnormality such as the crack. The aforementioned process is disclosed in JP-A-8-327586, for example.
In the case where the negative voltage is applied to the oxygen sensor in the generally employed process, a certain length of time (about several seconds) will be taken until the current value output from the oxygen sensor is stabilized. Accordingly, it is difficult for the aforementioned process to detect the abnormality in the oxygen sensor rapidly with good response. Execution of the abnormality detection process (for several seconds) as aforementioned may hinder the air fuel ratio control executed by the oxygen sensor.